1. Field of the Invention
The invention relates to a differential gear unit, and more particularly, to a lubricating structure of a differential gear unit.
2. Description of the Related Art
A differential gear unit incorporated into a drive train of a vehicle such as an automobile, for example, has a pair of side gears and a pair of pinions that are in mesh with each other, and a differential case that rotatably supports these gears. The pair of side gears and the pair of pinions rotate in mesh with each other, and a pair of drive shafts that support the pair of side gears rotate relative to the differential case. Also, the pinions rotate relative to a pinion shaft that is supported by the differential case, the differential case rotates relative to a differential housing in which the differential case is housed, and a bearing is arranged between the differential case and the differential housing. Accordingly, parts that rotate relative to one another during differential rotation of the differential gear unit need to be lubricated by lubricating oil to reduce the frictional resistance of those parts. A variety of lubricating structures have been proposed for this.
For example, one well-known lubricating structure for a differential gear unit is configured to lead lubricating oil to a chamber space between the differential case and the differential housing, or to a chamber space around one drive shaft through a lubrication hole by a lubrication supply system. Another well-known lubricating structure has a spiral groove provided on an outside surface of one drive shaft or on an inside surface of a boss portion of a differential case that rotatably supports the one drive shaft, and is configured to supply lubricating oil into the differential case by the spiral groove.
In particular, Japanese Patent Application Publication No, 2010-144904 (JP 2010-144904 A) describes a lubricating structure of a differential gear unit, that has a ring plate in addition to a lubrication hole and a spiral groove. The ring plate has a tongue portion that leads lubricating oil that flows into a chamber space around a drive shaft through the lubrication hole provided in a differential housing to between the drive shaft and a boss portion of a differential case.
According to this related lubricating structure, lubricating oil is able to be supplied to the inside of the differential case and to between the drive shaft and the boss portion well compared with when the lubrication hole and the spiral groove are not provided or when the ring plate that has the tongue portion is not provided.
Typically in a differential gear unit, the drive shaft must be well supported, so the clearance between the drive shaft and the boss portion of the differential case is extremely small. Similarly, the clearance between the shaft portion of a side gear fixed to an inside end of the drive shaft and the boss portion of the differential case is also small.
Therefore, the smallest passage sectional area of a lubricating oil flow path that leads from the chamber space around the drive shaft to the chamber space inside the differential case through between the drive shaft and the boss portion of the differential case is smaller than the smallest passage sectional area of the lubrication hole. Therefore, a flow path resistance of the lubricating oil flow path that leads from the chamber space around the drive shaft to the chamber space inside the differential case through between the drive shaft and the boss portion of the differential case is high, so the flowrate of the lubricating oil that flows through this lubricating oil flow path is restricted. Thus, when the differential gear unit operates under severe operating conditions, there may not be sufficient lubrication between the drive shaft and the boss portion of the differential case, or between the shaft portion of the side gear and the boss portion of the differential case or the like. Therefore, portions where lubrication may be insufficient need to be plated with plating into which a solid lubricant has been mixed, or polished to a high degree such that their surface roughness becomes low.